Production of potassium azide by double replacement

ABSTRACT

POTASSIUM AZIDE IS PRODUCED BY PRECIPITATION FROM A SOLUTION OF DISSOLVED SODIUM AZIDE AND POTASSIUM CARBONATE. BY CONDUCTING PRECIPITATIONS OF POTASSIUM AZIDE AND BY-PRODUCT SODIUM CARBONATE AT TWO DIFFERENT TEMPERATURES, A CYCLIC PROCESS MAY BE ACHIEVED.

April 9, 1974 w. K. SNEAD ETAL 3,803,296

PRODUCTION OF POTASSIUM A'ZIDE BY DOUBLE REPLACEMENT Filed March 27, 1969 ll Sheets-Sheet 1 K CO B I I I I I r I KN FIG. 1

Na ca D' A NQ,N3

INVENTORS WILL/AM K. SNEAD 18055127 5. MkfifivY D" 14 w FIG. 2. Amiga/ is April 9, 1974 w SNEAD ETAL PRODUCTION OF POTASSIUM AZIDE BY DOUBLE REPLACEMENT ll Sheets-Sheet 2 Filed March 27, 1969 INVENTOR5 NmN FIG. 3

W 5 AF. 5k 3 K 6 7 B L5 B ow BY w gr w.

ATTORN E Y5 April 9, 1974 w, SNEAD ET AL 3,803,296

PRODUCTION OF POTASSIUM AZIDE BY DOUBLE REPLACEMENT Filed March 27, 1969 ll Sheets-Sheet I5 N MOLES OF H20 PER EQUIVALENT SALTS INVENTORS WILL/AM K. SNEAD 085,427 5. M 61955 W ATTORNEYS April 9, 1974 PRODUCTION OF POTXSSIUM AZIDE BY DOUBLE REPLACEMENT Filed March 27, 1969 W. K. SNEAD AL 11 Sheets-Sheet 4 H 0 PER.

EQUIVALENT SALTS FIG. 5

FIGJ:

MWJM NW3 ATTORNEYS April 9, 1974 SNEAD ETAL 3,803,296

PRODUCTION OF POTASSIUM AZIDE BY DOUBLE REPLACEMENT Filed March 27, 1969 11 Sheets-Sheet 5 Z N 9.0 2 a 31 Y Z "8 O I .*7.0 1 I m I F 1 -6.0 m I '0 E r -5.0 I I a 1 a" F "4.0 5

NA CO 'H O l I I o I I 1 Q INVENTORS WWW ATTORNEYS April 9, 1974 W. K. SNEAD ET AL PRODUCTION OF POTASSIUM AZIDE BY DOUBLE REPLACEMENT Filed March 27 ll Sheets-Sheet 5 FIG. 65

M Wm

ATTORNEY;

April 9, 1974 SNEAD ETAL 3,803,296

PRODUCTION OF POTASSIUM AZIDE BY DOUBLE REPLACEMENT Filed March 27, 1969 ll Sheets-Sheet 7 KzCQ; B 1 I I l I L KN El 80C F c Q C uwzcos l l I I D1 I I D I INVENTORS W/u/AM K. sis/5A0 leoelser .5 M GREEVY ATTORNEYS April 9, 1974 w, SNEAD ETAL PRODUCTION OI" POTASSIUM AZIDE BY DOUBLE REPLACEMENT ll Sheets-Sheet 8 Filed March 27. 1969 O u n n 3 H M J w I 4 .H Han W 0 o O I o o w v H ll 3 E O c I Z l k E N q v 3 IL 0 C FIG. 7A

ATTORNEYS April 9, 1974 Filed March 27, 1969 -W- K. SNEAD ET L 11 Shets-Sheet a l KZCO B B l l 1 A! 4 I 2 F F Mg F N z s I I Du 1 D, F r FIG. 8

I INVENTORS W/LL/AM K. SWEAD Rdfil'fiTE- MGRfEVY ATTORNEYS PRODUCTION OF POTASSIUM AZIDE BY DOUBLE REPLACEMENT ll Sheets-Sheet 10 Filed March 27, 1969 KOrmm INVENTORS PRODUCTION OF POTASSIUM AZIDE BY DOUBLE REPLACEMENT Filed March 2'7, 1969 ll Sheets-Sheet S Y 5 R 0 v a w 5:08: o A E wfiwmmfi $55 92 M W m m 20.53.53 mag Pr ms w I K MW A United States Patent O 3,803,296 PRODUCTION OF POTASSIUM AZIDE BY DOUBLE REPLACEMENT William K. Snead, Wheeling, and Robert E. McGreevy,

New Martinsville, W. Va., assignors to PPG Industries,

Inc., Pittsburgh, Pa.

Filed Mar. 27, 1969, Ser. No. 810,980 Int. Cl. C01b 21/08 US. Cl. 423-410 25 Claims ABSTRACT OF THE DISCLOSURE Potassium azide is produced by precipitation from a solution of dissolved sodium azide and potassium carbonate. By conducting precipitations of potassium azide and by-product sodium carbonate at two diiferent temperatures, a cyclic process may be achieved.

SUMMARY OF THE INVENTION This invention deals with the production of potassium azide. It more particularly relates to the production of potassium azide from sodium azide and potassium carbonate by double replacement. An additional feature of the invention allows potassium azide to be produced in a cyclic process.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a J'zinecke plan projection of a portion of the NaN -K CO reciprocal salt pair system at 25 C.

FIG. 2 is a superimposition of the Janecke plan projections of portions of the NaN -K CO reciprocal salt pair system for the 25 C. and 80 C. isotherms.

FIG. 3 is a Janecke projection of a portion of the NaN -K CO reciprocal salt pair system with water at 25 C.

FIG. 4 is a Ianecke projection of a portion of the NaN -K CO reciprocal salt pair system with water at 80 C.

FIG. 5 is the water elevation of a portion of the Janecke projection for the NaN -K CO reciprocal salt pair system at 25 C.

FIG. 6 is similar to FIG. 2, showing a process according to this invention.

FIG. 6A is a superimposition of the water elevations of portions of the J'anecke projections for the NaN -K CO reciprocal salt pair system at 25 C. and 80 C. further showing the process of FIG. 6.

FIG. 6B is an enlargement of the central portion of FIG. 6A.

FIG. 7 is similar to FIG. 6, showing a modified process according to this invention.

' FIG. 7A is similar to FIG. 6A, further showing the modified process of FIG. 7.

FIG. 8 is similar to FIG. 6, showing another modified DETAILED DESCRIPTION Potassium azide may be produced by precipitating the same from an aqueous solution containing significant quantities of sodium ions, potassium ions, azide ions and carbonate ions. A quantity is significant in this respect when each of the four enumerated ions is present in solution at a concentration of at least 1 percent by weight of the total anhydrous salts comprising the solution. Thus, each of these ions is present in solution in more than mere trace or contaminating amounts. Such a solution may be formed from sodium azide, potassium carbonate and water. Potassium azide is then precipitated from the solution. Whether the enumerated ions are simple or compleX or whether they are truly ionic in the classical sense, is of secondary consideration. The important characteristic of these substances is that they are able to undergo rearrangement from the parent feed materials to form potassium azide and sodium carbonate.

When referring to reciprocal salt pair systems, it is convenient to represent the system by means of the Janecke projection. General reference in this regard may be made to Purdon and Slater, Aqueous Solution and the Phase Diagram, Edward Arnold & Co., London (1946).

Referring now to FIG. 1, there is shown a Janecke projection of a portion of the aqueous system at 25 C. where each corner of the square represents a pure salt. Each salt of a salt pair is positioned at the end of a diagonal opposite from the other. Each corner not only represents a pure salt, but implies a definite quantity of that salt, usually, but not necessarily, one chemical equivalent. Chemical equivalents are used rather than moles or grams since the above equation indicates that one molecule of sodium azide or potassium azide is equivalent to one-half molecule of sodium carbonate or potassium carbonate.

If the corners are taken as representing one equivalent of the pure substance, any point within the diagram represents one equivalent of mixed salts with the amount of water unspecified. Since one equivalent of any salt will yield two equivalents of ions, the sum of the cations equals one equivalent and the sum of the anions also equals one equivalent.

Each side of the square is a straight line connecting compounds having a common ion. The top horizontal edge of FIG. 1, for example, connects the corners representing potassium carbonate and potassium azide. The common ion in this case is the potassium ion. For any point on the diagram, the amount of potassium ion present in solution may be found by passing a vertical line through the point and applying the well-known principle of the lever rule, where the top horizontal edge of the square indicates that all the cations are potassium ions. According to the lever rule, the fraction of cations which are potassium ions is represented by the proportion ps/ rs and the fraction of cations which are sodium ions is given by pr/rs. The rules derives its name from an analogy with the leverarm principle of mechanics. According to this analogy, if all of the potassium ions were placed at point r on beam rs, all of the sodium ions placed at point s and the fulcrum positioned at point p, the hypothetical beam rs will be in balance if the amounts of potassium ions and sodium ions placed at points r and s respectively are inversely proportional to their distances from the fulcrum. Thus, for point p on the diagram, the concentration of potassium ions is represented by the ratio ps/ rs multiplied by the quantity of salt represented by a corner. If the quantity of salt represented by each corner is one chemical equivalent, the

potassium concentration is (ps) (1.0) /rs=0.7 equivalent.

(1.0) /rs=3.0 equivalent. The concentration of anions.

may be found in an analogous manner by measuring the relative position of point p from the two vertical edges of the square. Moreover, since the basis is one equivalent of salts, scales may be positioned at the edge of the square indicating equivalent fractions. While the level rule has been described with respect to ions, it is equally applicable to compounds, salt mixtures and solutions which appear on the projection.

While a point on the diagram may be represented in terms, of its ions, it is often convenient to express its position in terms of salts. Since any mixture of four possible salts can ge expressed in terms of three salts, any point in the square may be regarded as being in an isosceles right triangle, or half a square. Point p is in the triangle Only two figures are required to fix the position of the point, and it is most convenient to take the figures for the salts at opposite ends of a diagonal, which in this case are KN and Na CO The amount of salt represented by the salt at the right angle of the triangle is equal to the total amount of salt represented by the system less the sum of the amounts of salt whose identities are represented by the salts at ends of the diagonal. In FIG. 1 the composition ofpoint p expressed in terms of three salts is (r-K CO (1.0) /r.r=0.6 equivalent KN (pr)(l.0) /rs=0.3 equivalent Na OO and equivalent K C It will be seen that the amount of the third component, K 00 is equal to (pw)(l.0)/ (rs) where pw is the distance from point p to the diagonal which is measured parallel to a side of the square. Even though the composition of point p may be expressed in terms of percentages of KN 'Na CO and K 00 it is, of course, understood that physically these salts need not be present as the salts per se, but may actually be present in some other form such as ions. For this reason, the composition of point p may be equally as well expressed in terms of the salts K CO NaN and KN since point p is also located within the triangle K CO -NaN -KN The principles are analogous to those just described.

In the absence of a water elevation or solid model, both of which will be more fully described hereafter, it is assumed that the water content of a point of the Janecke projection is such that the solution is saturated. Curves AE, BE, EF, CF and DF are then placed upon the square, which divide it into four areas known as fields. Any point within a field represents the composition of a solution which is saturated with respect to one salt. The identity of the salt is indicated by the corner, but the degree of hydration, if any, is undefined. Thus, the solution point p is saturated with respect to potassium azide.

A point on any of the curves AE, BE, EF, CF 0r DF, represents a solution saturated with respect to two salts. These two salts are indicated on an anhydrous basis by the two corners of the adjacent fields. Point t, for example, is saturated with respect to sodium carbonate and potassium azide. The points E and F are quaternary invariant points which represent solutions saturated with respect to three salts represented by the corners of the adjacent fields. Thus, the point F is saturated with respect to sodium carbonate, sodium azide and potassium azide.

When water is isothermally evaporated from a solution saturated with respect to one simple salt or one hydrated simple salt, that salt, which is represented. by the corner of the field, precipitates and the composition of the solution moves away from the corner of the field along a straight line passing through the corner of the field and the point representing the initial solution composition. As water is isothermally evaporated from the saturated solu- 7 tion of point p, potassium azide precipitates and the solution composition moves to point u. The fraction of the total salts precipitated as potassium azide is up/u-KN in accordance with the lever rule. Additional water may be removed with the concomitant precipitation of potassium azide until the solution composition reaches point t. The lever rule may again be applied to determine the fraction of total salts precipitating as potassium azide. Any further removal of water will result in the precipitation of both potassium azide and sodium carbonate while the solution composition moves along the curve EE towards point E. Upon reaching point E, the solution composition Will remain constant as potassium azide, sodium carbonate and potassium carbonate are precipitated from solution. Continued isothermal evaporation of water will result in the dryingof the solution. During the entire isothermal evaporation procedure, the over-all composition of the complex, irrespective of the number of phases present, will be represented by point p. This point will continue to represent the over-all composition until one or more salts are added to or removed from the system.

FIG. 2 shows the J anecke projection of FIG; 1' having a similar projection for the C. isotherm super-imposed upon it. With the change in temperature from 25 C. to 80 C., A has moved to A", B to B, C to C, D to Df, E to E and F to F. Precipitations may be represented in a manner analogous to the procedure using the 25 C. isotherm.

FIG. 3 represents the basic Janecke projection of FIG. 1, having a scale representing moles of water per equivalent of anhydrous salts projected vertically from the base plane which represents the salts. The resulting diagram may be thought of as representing a three-dimensional solid, but the plan has not been shown foreshortened in the interests of ease of drawing and ease of comparison with the plan projection of FIG. 1. Pure water is represented by a point infinitely distant from the plan. Above the plan are shown the surface representing the water content for a saturated solution. Any point on the saturation" surface represents a saturated solution and any point 'above a saturation surface represents an unsaturated solution. A point between the saturation surface and the plan represents the over-all composition of a system having asolid phase and a saturated solution. A straight line connects the point representing the solid phase composition, the point representing the overall composition (the complex) and the point representing the saturated solution. The lever rule may be applied to determine the relative amounts of solid phase and saturated solution. Any point on the plan represents anhydrous salts.

Upon the isothermal evaporation of water from an unsaturated starting solution, p the solution point, will descend vertically until it reaches the saturation surface at point p Further isothermal evaporation of water will result in the precipitation of potassium azide and the movement of the solution point along the curve N1 on the saturation surface from p towards t Upon reaching" point t,, the continued evaporation of water causes the point representing the solution to move along curve E ,.F to E with the precipitation of potassium azide and sodium carbonate. As additional water is removed, the solution composition remains at E and the three salts of'the adjacent fields'precipitate. This precipitation continues until the solution dries. Similar diagrams may, of course, be constructed for other isotherms. Representative of these is FIG. 4, constructed for the80 C. isotherm.

7 Instead of using a solid model or a diagram representing the same, it is often more convenient to use an elevation projection illustrating the water content. FIG. 5 is the water elevation of a portion of the NaN -K CO reciprocal salt pair system at 25 C. looking in the direction shown by the arrow in FIG. 3. Again, isothermally evaporating water fr m unsaturated soluti P2: the

quence of events is the same. The point representing the solution composition moves from 1 to 17 from p to t with the precipitation of potassium azide, from 1'; to E with the precipitation of potassium azide and sodium carbonate, from E to E with the precipitation of potassium azide, sodium carbonate and potassium carbonate. In order to exemplify more clearly the basic principles involved, FIGS. 3, 4 and 5 do not show the formation of hydrates.

FIGS. 6 and 6A show the plan and water elevation, respectively, of portions of the 25 C. and 80 C. isotherms and may be used to illustrate the phenomenon of cooling. FIG. 6B is an enlargement of the central portion of FIG. 6A and more clearly shows the relationship of several of the points in this region of FIG. 6A.

The effect of cooling solution a which is saturated with respect to potassium azide and sodium carbonate, may best be demonstrated by considering a hypothetical process where water is added at 80 C. to saturated solution a until the solution point is at some point a above the curve b N. Upon cooling to 25 C., the solution point remains at a since no material precipitates or is removed from the system. When Water is isothermally removed at 25 C., the solution point descends vertically until it reaches the curve b N at a The solution is now saturated with respect to potassium azide. With the continued removal of water, potassium azide precipitates and the solution point moves from u towards d The over-all composition point continues to descend vertically since only water is being removed from the system. The relationship of the solution point, the over-all composition point and the corner representing potassium azide is such that a straight line connects all three points. When the overall composition reaches point a the solution point reaches d At this stage of the hypothetical process, the same amount of water that was added to solution a has been removed, returning the over-all composition to point a In the interim, however, the system was cooled and potassium azide precipitated. While it is permissible to add and then remove the water in an actual process, it is more convenient to simply cool solution a from 80 C. to 25 C. with the precipitation of potassium azide. The result is the same: over-all composition a contains precipitated potassium azide and saturated solution d FIGS. 6 and 6A may also be used to illustrate a cyclic process for the production of potassium azide from sodium azide and potassium carbonate. Sodium azide and potassium carbonate are dissolved in the proper proportions to form a saturated solution at 80 C. represented by the point e Water is then isothermally evaporated, moving the solution point from e to a with the deposition of sodium carbonate monohydrate. The precipitate is removed from the system, leaving solution a Centrifugation or filtration may be used for this purpose. Solution a is then cooled from 80 C. to 25 C., resulting in solution a and precipitated potassium azide. Water is isothermally evaporated at 25 C. to deposit more potassium azide until the solution point reaches b The solid potassium azide is next removed from the solution. Centrifugation or filtration is useful for this purpose. Solution b is then heated to 80 C. A mixture of sodium azide and potassium carbonate, represented by point 0 is added to solution b to form complex at 80 C. Water is added to complex 0 which is saturated with respect to sodium carbonate monohydrate to form saturated solution 0 A slight excess of water may be added to form solution 0 to insure dissolution of all of the salt. Water is isothermally evaporated at 80 C. to deposit sodium carbonate monohydrate until the solution composition reaches point a The sodium carbonate monohydrate is then removed from the system. Solution al may again be cooled to .25 C. and the cycle repeated as often as desired.

Since the Janecke projection represents equilibrium conditions, it does not indicate the rates of transformation during various stages of the process. When, for example, mixture 0 is added to solution b; at 80 C. to form complex 0 time must be allowed for a portion ofthe solid phase to dissolve and for the remaining portion to be converted to sodium carbonate monohydrate. If left long enough to assure equilibrium conditions, complex c would provide solution a; and solid sodium carbonate monohydrate. The time lag may be substantially reduced by dissolving mixture 0 in the water necessary to form saturated solution 0 This solution is then added to solution b to form unsaturated solution c Water is isothermally removed from solution c to formsaturated solution 0 The precipitation of sodium carbonate monohydrate may then be accomplished with the further evaporation of water as heretofore described.

In order to insure the complete dissolution of the proper amount of mixture 0, it is preferable to use a slight excess of water to form unsaturated solution 0 When this solution is mixed with solution b the unsaturated solution 0 is formed. The isothermal evaporation of water at C. will cause the solution point to move from c to 0 from whence the precipitation may begin. I

It is preferred that the temperature of feed solution 0 or 0 and the temperature of solution b; be about 80 C. when the solutions are mixed. This is not strictly necessary, however. The temperature of the feed solution may be greater or lesser than that of solution b Of course, if the temperature of a solution is cooled below the point at which it becomes saturated, salt will ordinarily precipitate. This is not the preferred mode of operation since long periods of time are often required to bring the mixture to equilibrium. If solutions of divergent temperatures are to be mixed, it is preferred that the temperatures and specific heats be such that the resultant temperature of the mixture be at the desired temperature for the evaporation of water, in this instance, about 80 C. Otherwise, heating or cooling of the mixture is required.

FIGS. 7 and 7A illustrate one modification of the basic process which may be used to advantage. When solution d is formed by the cooling of solution al additional potassium azide may be precipitated by isothermally evaporating water to move the solution point from d to b asheretofore described. The isothermal removal of water at 25 C. is, of course, a slow process which may be accelerated by the application of a vacuum. This requires the expense of a vacuum system. Moreover, the additional amount of potassium azide precipitated by this step is moderate. It may be more advantageous to forego the immediate recovery of this amount of potassium azide. Once the precipitated potassium azide has been removed from solution d the solution may be heated to 80 C. and mixture 0 added to form complex f Water is then added to form saturated solution f or more preferably, unsaturated solution f Alternatively, solution 0 comprising salt mixture 0 and water, may be added to form solution f In order to assure complete dissolution of salt mixture 0 and the purity of the sodium carbonate monohydrate to be precipitated, it is preferable to add unsaturated solution 1%,. Water may then be isothermally removed at 80C. to bring the solution point to h. Continued isothermal evaporation of water moves the solution point to a with the precipitation of sodium carbonate monohydrate. The precipitate may then be separated and solution a cooled to precipitate potassium azide. Additional cycles may be performed as desired.

Obviously, this modification may be practiced only in part by isothermally removing only a portion of the water required to move the solution point from d to point b before adding mixture 0 or a solution thereof.

Another modification, which is the preferred mode of operating the system, is shown in FIGS. 8 and 8A. Here salts are precipitated only until the solution point approaches the lines of saturation with respect .to two salts rather than touches them. This insures the purity of the salt by reducing the possibility for a precipitation of both salts due to short term fluctuations in the sur- .7 rounding-conditions. Thus, solution g which approaches but does not coincide with pointa may be cooled to .form .solid potassium azide and saturated solution k which lies. on curve m= N. Water is next isothermally removedto reach point it, which approaches but does not coincide with point m 'If desired, water may be ,jremovedlfrom solution g as it is cooled, in which case the solution ,point reaches point k without passing through point k Care must be exercised, however, to adjust the. relative rates of evaporation and cooling so ,ast'o. avoid the precipitation of two salts unless salt purity is of little consequence.

Upon reach ng point h, .the precipitated potassium .azide is then removed from the system. Solution I1 is ,next heatedlto 80 C. Mixture'o is then added to form complex i5. Water is added to form saturated solution i or if water is in excess, unsaturated solution i Again, it is preferred to add a solution of mixture rather than adding the solid salts. Saturated solution 0 may be added "to solution hi to form unsaturated solution f In order to assure that the salt mixture 0 is in solution, it is preferred to add unsaturated solution 0 to solution k to give unsaturatedsolution i Wateris then isothermally -evaporatediat 80 C. from solution jg or solution i as the case rnay be, to produce saturated solution 1' Fur ther isothermal evaporation precipitates sodium carbonate monohydrate while moving the solution point to point g;. The cycle may be repeated as often as desired.

It will be apparent that the cyclic processas a whole .or any of its individual steps may be operated either continuously or intermittently. FIG. 9 illustrates a continuous process which may be used in accordance with this invention. A solution containing significant quantities of potassium ions, sodium ions, azide ions and carbonate ions is introduced into cooling crystallizer 1 through line 2. Mother liquor and the feed solution are circulated through line 4, and heat exchanger 6 by pump 8. Coolant is circulated through the heat exchanger to cool the circulating mother liquor and feed solution to about 25 C. The mother liquor, supersaturated with respect to potassium azide, is introducedto .a bed of potassium azide crystals through line 9, vapor head 10 and leg 11. A vacuum may be applied to the vapor head if vapor is to be removed. Magma is withdrawn from the crystallizer through line 12 by pump 14. The magma is introduced to a centrifuge 16 where mother liquor is removedthrough line 18 and the crystals are removed through line 20. The crystals may be washed if desired, but this is not ordinarily necessary where the. potassium azide is to be used for agricultural purposes. The crystals are then dried in dryer 22 while vapors are removed through line 24. Mother liquor is introduced to heat exchanger 26 where it is heated to about 80 C. The warm mother liquor then proceeds to mix tank 28 where it is combined with an aqueous solution of sodium azide and potassium carbonate which is introduced to mix tank 28 through line 30. The solution is withdrawn from mix tank 28, fed through line 32 and combinedwith the circulating mother liquor in evaporator-crystallizer 34. Circulation is provided by the pump 36. The circulating liquidis' heated in heat exchanger 38 so the vapor will flash off in vapor head 40. A vacuum may be applied to the vapor head or not, as desired. The solution, supersaturated with respect to sodium carbonate monohydrate is passed through leg 42 and introduced into a bed of sodium carbonate monohydrate crystals where sodium carbonate monohydrate precipitates. Magma is withdrawn from evaporator-crystallizer 34 through line 44 by pump 46 and introduced to centrifuge 48. 'Mother li quoris removed from centrifuge 48 throughline 2 and returned to cooling crystallizer 1. The azide remaining with the crystals is removed by wash water introduced through line 50. Azide-containing wash water is removed through line 52. This wash water may be sent to azide destruction and disposal through line 53, but it is preferably forwarded to evaporator 54 through line 56. Sodium carbonate crystals 'are removed from centrifuge 48 through line 57 and sent to dryer 59 where Water vapor is removed through line 61. The wash water fed to evaporator 54 is mixed with circulating liquor circulated through heat exchanger 58 by pump 60.

Vapor is flashed from the superheated liquid in evaporator 54. Concentratedsolution containing azide and carbonate ions is then fed through line 64 to dissolver 66 Where it is mixed with sodium azide and potassium carbonate. These solids are dissolved preferably by agitation While the solvent is circulated through heat exchanger 68 by pump 70. Solution at C. is removed from the dissolver 66 and introduced to mix tank 28 through line 30. The evaporator may be eliminated, if desired, provided that the amount of water used to wash the sodium carbonate crystals does not exceed the water removed from other regions of the system. Should the system require make-up water, it may conveniently be added to dissolver 66.

By properly regulating the concentrations of the solutions in accordance with Ianeckes projection of the NaN K CO reciprocal salt pair system, and by. regulating the water balance of the system, a continuous process for the production of potassium azide is provided. This process also allows the byproduct sodium carbonate to be used for many purposes without further purification. Of course, the sodium carbonate leaving the dryer 59 maybe anhydrous or in the form of the monohydrate, depending upon the amount of water. being removed by the dryer. Ordinarily, only trace amounts of azide remain in this material.

The feed mixture 0 has been discussed with reference to the anhydrous salts, but it is often convenient to add mixtures containing hydrated salts such as potassium carbonate sesquihydrate or potassium carbonate dihydrate. The point representing the particular hydrate con taining salt mixture which is used may be plotted on the J'anecke projection according to the principles heretofore described. Further lever rule calculations are then based upon this point.

Mixture 0 may be prepared indirectly rather than by the direct mixing of sodium azide and potassium carbonate. Thus, sodium azide and potassium hydroxide may be heated to form a melt into which carbon dioxide is introduced. The carbonated melt may then be cooled and introduced to the metathesis system with or without water in the form of anhydrous salt, hydrate or solution.

, Carbonation may also be accomplished in aqueous solution. Thus, sodium azide and potassium hydroxide are dissolved in water and carbon dioxide is introduced to the solution. The salts of the carbonated solution are then introduced to the metathesis system in any of the forms described above. Alternatively, sodium azide and potassium hydroxide may be added to the metathesis system and carbonated in situ. 7 When concentrated wash water is added to the system, the following procedure can be used to determine the location of the point representing the salt mixture added to the system. If the point representing the concentrated wash water is plotted on the diagram, a straight line conmeeting this point with the point representing the feed salt-mixture will pass through the point representing the composition of a mixture, of the two. The precise location offthe point on the linemay be found by applying the lever rule. When this point is ascertained, it may be used in conjunction with the point representing the mother liquor from the potassium azide precipitation to find the composition of the resulting mixture by application of the lever rule.

While the system has been described on the basis of only four ions, it is, of course, possible to add additional ions or compounds to the solution. The effect of these additional ions or compounds upon the system depends upon their identity and concentration. Since these effects '9 cannot ordinarily be predicted with certainty, empericalmethods are used in their determination. Foreign ions or compounds will occasionally cause the precipitation of double salts. This may 'be permitted where it is not neces-' sary to obtain a product pure with respect to one salt.

: equivalents (98.39 grams) of salts in solution as solution "c S till at'80 (L296 moles (53.34 grams) ofwater are isothermally removed precipitating 0.480 equivalent I(25.44 grains) of sodium carbonate as the monohydrate. Since-0.24 mole (4.32 grams) of water is precipitated The invention has been shown using particular points as water of hydration, 29.76 grams of sodium carbonate on the J'einecke projection, but it will be understood that monohydrate is precipitated. This sodium carbonate monothe points used may be varied upon the diagram. This hydrate is then removed, leaving 1.000 equivalent (72.95 will change the lengths of the lever arms for the various grams) of salts as solution a The separated sodium carcalculations involving the lever rule. 10 bonate monohydrate is washed and dried. The separated The basic principles of the present invention have been potassium azide is dried. The cycle is summarized in incorporated by way of example in the following specific Table II. Mother liquor left entrained with the crystals embodiments. In these examples, the location of the pertiis deemed to be negligible.

TALBE II Equivalent; Moles Grams Weight percent KN3 NazCOa NEN: H2O KN; NBzCOa NaN; H1O KNB Nazcoa NaNa H2O i ggg t t sgg aagg ig sg g 0.680 0.251 0.069 4.02 55.16 13.30 4.40 72.44 67.94 a 9.15 3.09 49.82

cooled to C. 26. 45

p egg l i g gg s i um d t25 0. 354 0.251 0.069 4. V 28. 71 10.30 4.49 gig;- 24.14 11.18 3.78 60.90 0.154 equivalent of KM precipitated; all precipitated KNaremoved. a 0.154 12.49

0.520 equivalent of salts as solution 01 which is 0.992.699.2095. 9223 are ?-.???....?-.e. at: 1 5-3? 4.57moles0fH2O added. 7 4. 57 82.35 Igggggzaflalboi5313?:853358203 c at80 C-.. 0. 680 0. 731 0.069 5.5% 55.16 38. 74 4.49 123 14 16.95 1.97 56 94 0.480 equivalent of NazCOa precipitated and v removed. 4 80 25.44 0.24 mole of H20 precipitated and removed as water ofhydration 0.24 4.32 Loooequivalentofsalts asso1utiona1at80 C 0.680 0.251 0.069 4.02 55.16 13. 4. 49 72.44 37.94 9.15 3.09 49.82

nent points on the Janecke projection are as shown-in EXAMPLE 11 Table I.

'IABLEI' I ""A cyclic process for producing potassium azide from Concentration expressed as equivalents of salt per equivalent sQdium azide and Potassium Carbonate is Performed which total saltsamo11nt of water e g e as moles p equivalent H may'be described with references to FIGS. 6, 6A and 6B. Using point a as the starting point for the cycle and Point NMC0= NBNT allowing point a 'to represent 1.000'equivalent (72.95 0.680 0.251 0.069 4.02 grams) of salts, the solution is cooled from 80 C. to

3:252 8 3% 8:3; 2:28 25 C while 0 326 equivalent (26.45 grams) of KN 0.500 0.500 0.000 7 0. 00 precip tate, leaving 0.674 equivalent (46.50 grams) of g igg 8:282 8:82? 2:21 salts as solution d Water in the amount of 1.37 moles 0. 459 0.494 0.047 5.20 (24.69 grams)" is isothermally evaporated at 25 C.,

thereby precipitating an additional 0.154 equivalent EXAMPLE 1 (12.49 ams) of KN; and leaving 0.520 equivalent (34.01 grams) of salts as solution b Precipitated potas- A cycllc Process for produclng potflsslum aZlde f sium azide in the amount of 0.480 equivalent (38.94 sodium azide and potassium carbonate 18 performed which grams) is removed f the system Solution is then may be described with reference to FIGS. 6, 6A and 6B; h d t 80 C, Next, a solution represented by point s g P 1 as'flle'stamng P 1 forfhe cycle and a, which comprises 0.480 equivalent (33.17 grams) of allowing P0111t 1 to represent 1-000 equivalent potassium carbonate, 0.480 equivalent (31.21 grams) of gr f salts. the q q cooledvfrom to sodium aiide and 5.13 moles 92.44 grams) of Water w e 9 3- -4 g of a is added, at 80 c.,'td solution b At this point there precipitate, 1 eavmg 0-674 eflmvalent (46-50 grams) of are. 1.480 equivalents (98.39: grams) .of saltsinsolution salts as 801111109 1- Water flm l of moles as solution 6,. Still .at 80 0., 0.56 mole 10.09 grams) (24-69 f f lsothermzfny evaporated 25 their of-wate'r-are isothermally removed,,resulting in solution by preclPltatmgan aidltwnal i eqlllvalent 6 Upon removing an additional 2.96 moles (53.34 grams) grams of KN and leaving 0.520 equivalent (34.01 grams) of W t 0a 0 0 480 1 t (25 4 f of salts as solution [1 Precipitated potassium azide in a er a cqujlva grams) 0 the amount of 0.480 equivalent (38.94 grams) is removed Sodumi i t i (4'32 grams) of water of from the system Solution b1--isthenheated 808C. hydration precipitate as sodium carbonate monohydrate. Next, 0.960 equivalent (64.38 grams) of a salt mixture Thls' psodlum mmhydrate lsrthen removed represented by point 0 which comprises 0.480 equivalent leavmg L000 equlvalent (72-95 i m of Salts as 50111 (33.17 grams) of potassium carbonate and 0.480 equivtion a .Tl1e separated sodium carbonate monohydrate is alent (31.21 grams) of sodiuma zi-de is added, together washedjanld dried- The'sfipafated' Potassium azide is driedwith 4.57 moles (82.35 grams) of water, to solution b The cycle is summarized in Table III Mother liquor left Upon obtaining equilibrium at 80 C., there are 1.480 75 entrained "with crystals is deemed to be negligible.

TABLE III Equivalent M 1 Grams Weight percent es KN 3 Name); NGN: H1 0 KN: N51100: NBN: H2O KN a N520 D3 NaNa H2O 1.000 equivalent of salts as solution or at 80 C.. 0. 680 0. 251 0.069 4. 02 55. 16 13. 30 4. 49 72. 44 37. 94 9. 3. 09 49. 82 0.326 equivalent of KN precipitated as solution 7 v cooled to 25 C. 0. 326 26. 45

0.674 equivalent of salts as solution d; at 25 C.-.-- 0. 354 0. 251 0. 069 4. 02 28. 71 13. 30 4. 49 72. 44 24. 14 11. 18 3. 78 60. 90 1.37 moles of HzO-removed at 25 C 1. 37 24. 69 0.154 equivalent KN; precipitated; all precipitated KN; removed 0. 154 p 12. 49

0.520 equivalent of salts as solution or which is Y a then heated to 80 C. 0. 200 0. 251 0. 069 2. 65 16. 22 13. 30 4. 49 47. 75 19. 84 16. 27 5. 50 58. 39 0.960 equivalent of salts and 5.13 moles of H added assolution O; 0. 480 0. 460 0. 00 5. 13 38. 94 25. 44 0. 00 92.44 24.- 83 16. 22 0. 00 58. 95

1.480 equivalent of salts as solution 04 at 80 C".-- 0. 680 0. 731 0. 069 7. 78 55. 16 38. 74 4. 49 140. 19 23. 12 16. 24 1. 88 58. 76 0.56 mole of H20 removed at 80 C 0. 56 10. 09

1.480 equivalent of salts as solution c; at 80 C 0. 680 0. 731 0. 069 7. 22 55. 16 38. 74 4. 49 130. 10 24. 14 16. 95 1. 97 56. 94 2.96 moles of H10 removed at 80 C 2. 96 53. 34 0.480 equivalent of NazCO; precipitated and removed. 0.430 25.44' 0.24 mole of H20 precipitated and removed as water of hydration 0. 24 k 4. 32

1.000 equivalent of salts sssolution a1at80 C 0. 680 s 0. 251 0. 069 4. 02 55. 16 13. 30 '4. 49 72 44 37. 94 9. 15 3. 09 49. 82

' While the invention has been described with reference to the 25 C. and 80 C. isotherms, other isotherms may be used' as desired. The maximum useful isotherm is the boiling point of the system under the environmental pressure involved while the minimum useful isotherm is the point at which the entire system freezes.

Water is the preferred solvent for the system. However, any material which is a solvent for potassium azide, sodium azide, potassium carbonate and sodium carbonate may be used provided potassium azide may be precipitated from solutions of these salts. The Iiinecke projection of the NaN -K CO reciprocal salt pair system may be constructed using a solvent otherthan water. Inspection of the projection and application of the principles applied in the aqueous system will indicate whether such a solvent is suitable for use in a cyclic or noncyclic process. Mixtures of solvents may also be used if desired.

It is to be understood that the terms potassium azide, sodium azide, potassium carbonate and sodium carbonate as used throughout the specification and claims are intended 'to includeanhydrous salts, hydrates, simple salts and double salts unless otherwise qualified.

What isclaimed is: p X 1. A method for producing potassium azide comprising:

(a) cooling a first aqueous solution containing at'leas t I one percent by weight of the total anhydrous salts present therein each of the following ions, sodium ions, azidc ions, and carbonate ions, toprecipitate potassium azide and to form a first mother liquor;

-(b) separating saidprecipitated potassium azide from said first mother liquor; v

(c) increasing the temperature of said first mother liquor to a second temperature which is higher than said first temperature; f I

(d) forming a second aqueous solution from sodium azide, potassium carbonate, water and 'said first motherliquor;

(e) removing water from said second aqueous solution' to precipitate sodium carbonate from said second aqueous solution to form 'a second mother 'liquonand r -(f) separating said precipitated sodium c said second mother liquor.

2. The method of claim 1 wherein said fir ture is about 25 C.

3. The method of cl perature is about 80C. 1 4 v v 4. The method of claim 1 wherein said. sodium carbonate is precipitated as sodium carbonate monohydrate.

5. The method of claim 1 wherein water is Qremovcd arbonate from st tempera aim 1 whereinsaid second tom-.

from said first aqueous solution while at least a portion of said potassium azide is precipitated.

6. The method of claim 1 wherein said second aqueous solution is formed at least in part by:

(a) dissolving said sodium azide and said potassium carbonate in said water to form a third aqueous solution;

(b) combining said third aqueous solution with said mother liquor; and

(c) removing water. I

7. The method of claim 1 wherein said separated sodium carbonate is washed to substantially remove an trained mother liquor.

8. The method of claim 7 wherein at least a portion of the wash liquor resulting from said washing is combinedwith said first mother liquor.

9. The method of claim- 8 wherein sodium azide and potassium carbonate are combined with said wash liquor prior to combining said wash liquor with said first mother liquor.

10. The mcthodof claim 8 wherein the concentration of azide ion present in said wash liquor is increased prior to combining said wash liquor with said first mother liquor.

11. The 'method of claim 10 wherein said concentration of azide ionin said wash liquor is accomplished by removing water from said wash liquor. I

12. The method of claim 1 wherein said precipitated potassium azide is separated from said first mother liquor by ccntrifugation.

-13. Themethod of claim 1 wherein said precipitated potassium azide is separated from said first mother liquor by.filtration. I

1 14. The method of claim 1 wherein said sodium carbonatc is separated from said second mother liquor by centrifugation. T

r 15. The method of claim 1 wherein said sodium carbonate. is separated from said second mother liquor by filtration. 16. A cyclic process for producing potassium azidc comprising: I

(a) at a first temperature, separating precipitated potassium azide from a first mother liquor obtained 7 from step e;. v I 3 (b) adding heat and combining (1) one equivalent of sodium azide per equivalent of said precipitated potassium azide separated -from said first mother liquor,

(2) one equivalent of potassium carbonate per equivalent of said precipitated potassium azide separated from said firstmothcr liquor,

13 (3) water, and (4) said first mother liquor to form an aqueous solution at a second temperature which is higher than said first temperature;

() removing water from said aqueous solution at said second temperature to precipitate one equivalent of sodium carbonate per equivalent of said precipitated potassium azide separated from said first mother liquor and to form a second mother liquor;

(d) separating said precipitated sodium carbonate from said second mother liquor; and

(e) cooling said separated second mother liquor to said first temperature to precipitate said potassium azide and to form said first mother liquor.

17. The method of claim 16 wherein said first temperature is about 25 C.

18. The method of claim 16 wherein said second temperature is about 80 C.

19. The method of claim 16 wherein said aqueous solution is formed at least in part by:

(a) dissolving said sodium azide and said potassium carbonate in water to form an aqueous feed solution; and

(b) combining said aqueous feed solution with said first mother liquor.

20. The method of claim 16 wherein said separated precipitated sodium carbonate is washed to substantially remove entrained mother liquor.

21. A cyclic process for producing potassium azide comprising:

(a) at a first temperature, separating precipitated potassium azide from a first mother liquor obtained from step e;

(b) adding heat and combining (1) one equivalent of sodium azide perequivalent of said precipitated potassium azide separated from said first mother liquor;

(2) one equivalent of potassium carbonate per equivalent of said precipitated potassium azide separated from said first mother liquor;

(3) water; and

14 (4) said first mother liquor to form an aqueous solution at a second temperature which is higher than said first temperature;

(0) removing water from said aqueous solution at said second temperature to precipitate one equivalent of sodium carbonate per equivalent of said precipitated potassium azide separated from said first mother liquor and to form a second mother liquor; (d) separating said precipitated sodium carbonate from said second mother liquor; and (e) cooling said separated second mother liquor to said first temperature and removing water to precipitate said potassium azide and to form said first mother liquor. 22. The method of claim 21 wherein said first temperature is about 25 C.

23. The method of claim 21 wherein said second temperature is about C.

24. The method of claim 21 wherein said aqueous solution is formed at least in part by:

(a) dissolving said sodium azide and said potassium carbonate in water to form an aqueous feed solution; and

(b) combining said aqueous feed solution with said first mother liquor.

25. The method of claim 21 wheren said separated precipitated sodium carbonate is washed to substantially remove entrained mother liquor.

References Cited UNITED STATES PATENTS 1,562,891 11/1925 Klopstock et a1 23-63 FOREIGN PATENTS 1,075,286 7/1967 England 23-191 OTHER REFERENCES Mellor, Inorganic & Theoretical Chemistry, vol. 8, supplement 2,.p. 16-18 (Longman Green 1967).

HOKE S. MILLER, Assistant Examiner UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N'o. 'VV3 ,803,296 Dated April 9, 1974 Inventor(s) William K. Snead et al.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 12, line 33, before "mother", insert first Column 12, line 36, before "sodium", insert precipitated Column 14, line 26, "wheren" should be wherein Signed and sealed this 29th day of October 1974.

(SEAL) Attest McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents USCOMM-DC 6037 B-PBD DRM Po-wso (10459) 7 U.$. GOVERNMENT PRINTING OFFICE: "II 0-3lU-3ll. 

